It is known for transmission errors to occur in the transmission path of a communications system and which act to impair the quality of the decoded speech signal in the receiver. Generally, attempts are made to avoid such transmission errors and in particular to protect speech coding parameters from such errors. For this purpose error correction coding is usually carried out in the channel coding for the transmitter, in connection with the channel coding, for the most important parameters to be sent. By means of the error correction coding the transmission errors which have been generated in the transmission path and have affected the error correction coded parameters can be detected and possibly corrected in the channel decoding carried out in the receiver. This kind of a method has been described in the article Eurospeech-91, D. Serene, "Frame substitution and adaptive post-filtering in speech coding", p. 595-598. Of transmission errors that cannot be corrected by the channel decoder have been formed in the transmission path the channel decoder indicates that the received speech frame is erroneous. In the following when reference is made to an erroneous or an error-free speech frame it is referring to the detection of each individual erroneous frame that has been carried out for each received speech frame on the basis of the error correction coding and possibly by some other reasoning.
The identification of erroneous or error-free speech frames can in some cases fail and create a situation in which the erroneous frame is interpreted as an error-free one or an error-free frame as an erroneous erre. Occasionally, there is a situation in which the transmission error is not detected and the erroneous speech coding parameters are used for decoding the speech signal which is particularly difficult and results in incorrect decoding. Due to the restricted capacity or information bandwidth of a typical transmission channel not all parameters can be protected by error correction coding, thus not all transmission errors can be detected in the receiver.
FIG. 1 shows a schematic block diagram for a well-known receiver of a communication system for transmitting speech signals. A received digital information frame 100 is introduced into the channel decoder 101. The speech coding parameters 102 according to whatever speech coding method is used are received from the channel decoder, and for each frame an indication of the level or error 103 i.e. information about whether an error has been detected in the received frame that the channel decoding has been unable to correct, is also received from the channel decoder. The parameters of the speech coder are introduced further into block 104, where a processing of the coding parameters is carried out. The processing typically contains some system for removing a bad frame or replacing it. The processed coding parameters 105 are finally introduced into the speech decoder 106, which on the basis thereof synthesizes the speech signal 107.
FIG. 2 shows more precisely the processing block 104 for the coding parameters of FIG. 1. This block is designated by the reference numeral 200 in FIG. 2. The speech coding parameters 201 of each received frame received from the channel decoder 101 are coupled directly to the output 203 if the error indicator 202 also introduced from the channel decoder 101 into the processing block 200 indicates that no uncorrected transmission error has been detected in the parameters of the received frame, nor can a transmission error in the received frame be detected in the parameters of the speech coder. The detection of transmission errors directly from the speech coding parameters is carried out in block 206 contained in the processing block of the coding parameters.
In the classifying block 207 the received frame is classified either as erroneous or error-free on the basis of various error indicators. On the basis of the result of the classification the switch 208 is controlled so that the received parameters 201 are introduced directly into the output 203, the switch in position 2, if the speech frame has been classified as error-free. In the case of an error being detected in the speech frame, the switch turns to position 1. As a result of this the parameters received from the replacement, or erroneous frames in the replacement black of erroneous frames 204 are coupled to output 203. In replacement block 204 the parameters of the erroneous speech frame are replaced with the parameters of the last received error-free frame or on the basis of these parameters with new parameters extrapolated from them by some suitable method. FIG. 2 displays a delay block 205 which shows the dependency of the parameters of each of the frames to be replaced on the parameters used in the former frame during the replacement procedure.
When information is received that the received speech frame is erroneous it is not worth while to bring the erroneous speech coder parameters to the speech decoder, but it is advisable to replace the lost speech frame by some suitable means such as described earlier. One well-known simple method to replace a lost speech frame is to silence the output completely instead of using the received erroneous parameters. This way of proceeding is not very good or practical because it causes breaks which clearly impair the quality of the synthesized speech signal.
Another well-known method, which is a better way than silencing the speech signal decoding, is to use some or all of the parameters of the last-received good speech frame instead of the lost parameters. The efficiency of this replacement procedure is based on the fact that most of the sounds of the speech signal last substantially longer than the frame length 20 ms typically used in speech coding and, thus, the properties of a speech signal do not vary very quickly. This kind of a procedure is described, for example, in UK patent application GE1-2,238,933. At its best, a speech signal corresponding fairly well to a lost speech frame can be produced by a replacement procedure. However, a drawback is that it cannot be repeated many times, for example, when several subsequently lost frames are being replaced the speech signal will gradually begin to sound unnatural and machine like. The duration of speech sounds is typically some hundreds of milliseconds at the most, so it is not reasonable to continue with the replacement procedure longer than this. when replacing several subsequent erroneous frames it is worth while carrying out the replacing by processing the parameters of the speech coder so that the speech signal fades out during a certain finite time. Hereby no sound remains continuously ringing even in the case when a long sequence of solely erroneous speech frames is received.
FIG. 3 shows a flow chart showing a replacement procedure of erroneous frames well-known in the field, which can be used in the system according to FIG. 2. The state 0 in the chart illustrates a situation where a frame classified as error-free has been received and hence no replacement procedure is needed, and the speech coding parameters of the received frame are introduced as such into the speech decoder, the switch of FIG. 2 being in position 2. State 0 is entered every time the received frame has not been indicated as erroneous and all error-free frames are introduced into the speech decoder. When after an error-free frame the first erroneous frame is received, the replacement procedure 301 for erroneous frames is started, whereby the switch of FIG. 2 turns to position 1. In case of subsequent erroneous frames the states 2, 3, etc. are entered. During the replacement procedure 301 the parameters of the last error-free frame are used to replace the frame received in an erroneous form. The replacement procedure by means of the parameters of the last error-free frame is carried out for M-1 subsequent erroneous frames at the most, where M is a positive integer. After this the state M is reached, in which parameters silencing the output completely are introduced into the speech decoder. In the replacement procedure the parameters are typically processed so that they produce a more attenuated speech signal for each subsequent erroneous frame replaced i.e. the higher the state number reached in the flow chart of FIG. 3 the greater the attenuation.
In the replacement procedure, usually, only the first speech frame to be replaced (i.e. when in state 1) is directly replaced with the parameters of the error-free frame as such and at least from the second frame to be replaced i.e. frame 2 on, the parameters are processed so that the synthesized speech signal begins to attenuate. The attenuation of a speech signal to be synthesized can be carried out using linear predictive coding of LPC type (Linear Predictive Coding), for instance, by attenuating the strength of the excitation signal from frame to frame.
The operation of the system can be described as follows using the markings
r.sub.rec =parameter which is received from the channel decoder and which transmits the amplitude of the speech signal to be synthesized, PA1 r.sub.dec =parameter which transmits the amplitude of the speech signal to be synthesized and which is fed into the speech decoder, PA1 r.sub.dec,prev =parameter which transmits the amplitude of the speech signal to be synthesized and which has been fed into the speech decoder during the previous speech frame PA1 step=a constant attenuating the replacement procedure during subsequent erroneous frames PA1 wherein the quality of a transmission connection is determined by means of the erroneous/error-free classification such that the quality deteriorates as the number of erroneous speech frames increases; PA1 the replacement of erroneous frames is depending on the quality of the transmission connection such that the duration and strength of the replacement is decreased while the quality deteriorates; and PA1 error-free frames are reduced in strength for a transmission connection of bad quality and in a transmission connection of good quality error-free frames are unattenuated. PA1 classifying a received speech frame as an error-free or erroneous speech frame; PA1 defining a first state D.sub.1 for replacing an erroneous speech frame with a speech frame corresponding to a previous error-free speech frame; PA1 defining further first sequential states D.sub.2 . . . D.sub.M corresponding to subsequent occurrences of erroneous speech frames and for replacing said subsequent erroneous speech frames with said speech frame corresponding to a previous error-free speech frame having sequentially increasing attenuation corresponding to a particular state D.sub.2 . . . D.sub.M ; and PA1 defining a second sequence of states U.sub.o. . . U.sub.M-1 respectively corresponding to an error-free frame being received during respective states D.sub.1. . . D.sub.M ; PA1 wherein a further occurrence of an error-free frame defines a state in a lower sequential position of said second sequence of states (U.sub.o. . . U.sub.M-1) and an occurrence of an erroneous frame defines a corresponding first state (D.sub.1. . . D.sub.M). PA1 classification means for classifying a received channel decoded speech frame as erroneous/error-free; PA1 a replacement means for replacing erroneous frames with frames corresponding to previously received error-free speech frames; PA1 coupling means, which in response to a control signal given by the classifying means couples either a replaced frame or an error-free speech frame to a speech decoder further comprising; PA1 a frame processing means, for receiving speech frames, processing received frames and coupling processed frames into the connecting means; PA1 a quality control means, responsive to said control signal for producing a signal indicative of the quality of a transmission connection, wherein PA1 the replacement means decreases the duration and strength of the replacement of erroneous frames while the quality of the transmission connection deteriorates compared with the replacement carried out during a good quality transmission connection; and PA1 the frame processing member processes the frames such that for a bad quality transmission connection the speech signal produced by the speech decoder is attenuated and for a good quality connection the frames are not processed by the frame processing means.
______________________________________ STATE 0: r.sub.dec = r.sub.rec STATE 1: r.sub.dec = r.sub.dec,prev STATE 2, . . . , STATE M-1 r.sub.dec = r.sub.dec,prev - step STATE M: silence output of speech decoder completely ______________________________________
In known speech coding systems which use the above-described replacement procedures of erroneous frames, a replacement procedure is carried out independently of the quality of the transmission connection. Thus, the replacement procedure is started as strongly for relatively good channel conditions as for very bad channel conditions. Hereby, some subsequent good frames that have arrived during a bad transmission connection in portions containing mainly erroneous frames start a replacement procedure continuing over several frames. Thus, a major part of the speech synthesized by a speech decoder during a transmission connection of bad quality is speech produced solely by means of the replacement procedure. By carrying out a strong replacement procedure when a bad transmission connection is prevailing is; is endeavoured to artificially lengthen the duration of the speech signal, although so few error-free speech frames are received there is insufficient information to initiate, on the whole, the replacement procedure on reasonable grounds and so that lost speech frames could be replaced by means of it well enough. During a bad transmission connection when a strong replacement procedure is started based on a single frame indicated as error-free, machine like sounds which the human sense of hearing cannot at all interpret as speech are caused in the speech signal deteriorating its quality.
In known speech coding systems the error-free speech frames are introduced into the speech decoder independent of the condition of the transmission connection. During a transmission connection of bad quality it is, however, not sensible, as far as the quality of the speech is concerned, to bring the parameters of a frame indicated as error-free as such directly to the speech decoder. However, it is worth while to soften in some suitable way the transition from the replacement procedure or instigate a complete attenuation of the output of the speech decoder to the use of speech frame parameters. This is due to several factors. Firstly, the activation of the speech decoder after a long transmission connection can produce a sudden change in the speech signal which impairs the quality of the speech. Although the parameters of the speech signal were error-free the speech decoder has, because of the break, lost information relevant to the previous frames of the .speech and used in decoding later speech and will not start decoding of the speech signal from a correct internal state. The internal state is affected e.g. by the values of the state variables of filters contained in the speech decoder, which after a long replacement procedure phase are no longer correct. Secondly, during a bad transmission connection it is more likely than otherwise that the error correction coding has failed in the frame interpreted as error-free and has given a wrong indication of the level of error in the speech frame- the received frame identified as error-free can contain bad transmission errors.